Unit used for `recombination_rate` when specifying a single value

[timothymillar]

Hi, thanks for msprime! The recombination_rate argument of sim_ancestry is a bit ambiguous when specifying a single value. The docstring states:

The rate of recombination along the sequence; can be either a single value (specifying a single rate over the entire sequence)...

I assume that the intention is to calculate the rate as Morgans / base positions?

[petrelharp]

That's right, yes. Well, it's a bit less obvious than that - it's "per unit time and per unit sequence"; and probably sequence is measured in units of bp, but that's not required.

Here's a PR; suggestions welcome: #2376
(but, maybe we just want to leave it how it was?)